1. Field of the Invention
The invention is related to the field of communications, and in particular, to systems and methods for identification of polarization-mode dispersion on a communication network.
2. Description of the Prior Art
Polarization-Mode Dispersion (PMD) is a dynamic pulse broadening phenomena. In a single-mode optical fiber, optical pulses propagating down the fiber will separate into two orthogonal modes of polarization that travel at different speeds. The relative amplitudes of these two pulses are determined by the state of polarization of the input pulse relative to the fiber's input principal states of polarization (PSP). The separation into the two orthogonal modes is caused by the non-uniformity of the core diameter of the fiber. This non-uniformity of the core diameter may be a result of imperfections in manufacturing, ambient temperatures, stress on the fiber, and/or movement of the fiber.
If the core has a perfectly circular cross-section, then both modes travel at the same speed over the same distance. Otherwise, one mode travels slower than the other mode resulting in a difference in group velocities (an effect called birefringence). Like chromatic dispersion, the difference in velocities between polarization modes is wavelength dependent. For PMD, the difference in velocity is also time dependent. The difference in propagation time, Δτ, experienced by the two polarization modes at a given wavelength is referred to as the differential group delay (DGD) with units in picoseconds (ps). It is well known that Δτ obeys a Maxwellian distribution. When the DGD in a fiber becomes excessively large, the receiver is unable to distinguish between a zero bit and a one bit, and bit errors occur eventually resulting in a PMD-induced outage.
PMD is a time varying stochastic effect. Identification, measurement, and compensation for PMD are difficult because of the time varying stochastic nature of PMD. A fiber operator may have an outage on a fiber that is unknowingly caused by PMD. Due to the time varying stochastic nature of PMD, the outage may resolve itself Thus, the fiber operator may have trouble tickets that are closed or left unresolved even after extensive activity by maintenance employees. PMD has traditionally been identified through direct measurement of PMD or other analog characteristics of the optical signal. Some prior systems have identified, measured, and compensated for PMD by measurement of DGD, PSP, or other control variables. One such system is disclosed in a pending U.S. patent application Ser. No. 09/500,092, entitled “Method And Apparatus To Compensate For Polarization Mode Dispersion,” filed on Feb. 8, 2000, which is incorporated herein by reference. Other prior systems monitor eye openings of the optical signal or perform other analog measurements of the optical signal to identify PMD. Another prior system measures the PMD, measures a bit error rate (BER), and correlates the PMD and BER to isolate errors due to the PMD. This system is disclosed in a pending United States Patent Application, entitled “Correlating Polarization Mode Dispersion and Bit Error Rate,” filed on Apr. 25, 2000 and having Ser. No. 09/558,448. One problem with these prior systems is the cost of identifying, measuring, and compensating for PMD can be expensive. What is needed is a system to monitor a communication system and identify PMD that is accurate and cost effective.